The present invention is directed to a gas laser comprising an optical resonator formed of two integrated mirrors, more than one longitudinal mode of the laser oscillation being excited therein during operation. Such gas lasers are described, for example, in Klaus Tradowsky "Laser", Vogel-Buchverlag Wuerzburg, 4th Edition, pages 44-45; 50. A laser resonator having more than one longitudinal mode is described on page 44, 45; and a laser having integrated mirrors is described on page 50. The "inner modulation" is described on pages 95-96 and a structure is shown wherein an electro-optical (birefringent) crystal, the modulator, and an analyzer are arranged in the beam path between the resonator mirrors. This, however, proceeds on the basis of a linearly polarized laser beam and the energy thereof is controlled with a phase rotation in the modulator.
When two longitudinal modes oscillating in polarized fashion are excited in the laser, their planes of oscillation being orthogonal to one another, then these laser oscillations tend to vary the angular position of the polarization planes of the longitudinal modes in what are frequently spontaneously occurring rotations around the axis of the laser beam. These rotations are undesired since--for example, for the purpose of a spatial separation of the two modes and for the formation of two coherent laser beams--the polarization planes should retain their angular position relative to a reference plane of the laser housing.